Attacks on fixed-apparatus quantum-key-distribution schemes

We consider quantum-key-distribution implementations in which the receiver's apparatus is fixed and does not depend on his choice of basis at each qubit transmission. We show that, although theoretical quantum key distribution is proven secure, such implementations are totally insecure against a strong eavesdropper that has one-time (single) access to the receiver's equipment. The attack we present here, the “fixed-apparatus attack,” causes a potential risk to the usefulness of several recent implementations.

Figure 1

Bob's measuring equipment in a fixed apparatus polarization-based BB84. PIBS$=$polarization independent beam splitter; PBS$=$polarization beam splitter; PR$=$polarization rotator.

Figure 2

Mach-Zehnder interferometer. (a) An input qubit. The time difference between the two incoming modes is identical to the difference between the two arms; (b) a vacuum state enters the second arm; (c) beam splitters; (d) $\Delta T$ time detour plus a phase shift $\phi$; (e) mirrors; (f) six output modes.